#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main()
{
    double m = 1, z = 1;
    double n = 1, s = 1, eps = 0, i = 1;
    scanf("%le", &eps);
    while (n >= eps)
    {
        z *= i;
        m *= i * 2 + 1;
        n = z / m;
        s += n;
        i++;
    }
    printf("PI = %.5lf", s * 2);

    return 0;
}



/*int n=1,i=1,s=0;
while(i<=100)
{
    n*=i;
    s+=n;
    i++;
}*/





